Liquefied natural gas (LNG) is obtained by liquefying natural gas, one of the fossil fuels, and LNG storage tanks may be divided into onshore storage tanks installed on the ground or buried in the ground and mobile storage tanks installed in vehicles such as automobiles and ships, depending on installation location.
Since LNG has a danger of explosion when exposed to impact and is kept under cryogenic conditions, a storage tank for LNG has a structure in which impact resistance and liquid tightness are firmly maintained.
In addition, in contrast to onshore storage tanks where sloshing of LNG is negligible, LNG storage tanks installed in automobiles and ships should be able to withstand mechanical stress due to the sloshing. However, since LNG storage tanks installed on ships equipped with measures against mechanical stress can also be used as onshore storage tanks, in the specification of the present invention, the structure of an LNG storage tank installed on a ship will be described as an example.
First, a typical LNG storage tank will be described with reference to FIG. 1.
FIG. 1 is a partial sectional view of a typical LNG storage tank.
Referring to FIG. 1, the LNG storage tank 10 includes: secondary heat insulating walls 22, 42 and primary heat insulating walls 24, 44 sequentially formed on a bottom surface of a hull; and secondary sealing walls 23, 43 interposed between the secondary heat insulating walls 22, 42 and primary heat insulating walls 24, 44 to seal a gap therebetween. In addition, a primary sealing wall 50 is disposed on the primary heat insulating walls 24, 44.
The LNG storage tank 10 includes: corner structures 20 disposed at inner corners of the storage tank; anchor structures 30 disposed at certain intervals on the bottom surface; and planar structures 40 interposed between the corner structures 20 or the anchor structures 30 to be slidable. Here, the corner structures 20, the anchor structures 30, and the planar structures 40 are preliminarily manufactured as unit modules, and then assembled into the storage tank 10, and the primary sealing wall 50 is disposed thereon to seal the heat insulating wall in a liquid-tight manner, thereby providing an inner space for storing LNG.
The corner structure 20, the anchor structure 30, and the planar structure 40 include the primary heat insulating walls 24, 34, 44, the secondary heat insulating walls 22, 32, 42, and the secondary sealing walls 23, 43, and are collectively referred to as heat insulating wall structures 20, 30, 40, respectively.
In the heat insulating wall structures 20, 30, 40, contact surfaces between the secondary sealing wall and the heat insulating wall of each of the unit modules are bonded together via adhesives. Generally, the secondary heat insulating walls 22, 42 are composed of polyurethane foam, which is an insulating material, and a plate attached under the polyurethane foam. The primary heat insulating walls 24, 44 are composed of polyurethane foam and a plate adhered thereto with an adhesive. The primary heat insulating walls 24, 34, 44 are disposed above the secondary heat insulating walls 22, 32, 42, respectively.
In addition, the secondary heat insulating wall 42 of the planar structure 40 is formed at a lower end thereof with a flange 42a, which is larger than the secondary heat insulating wall 42. The flange 42a is inserted into a groove formed in a lower end of the anchor structure 30 to be slidable to some extent.
In the illustrated example, each of the anchor structures 30 includes an anchor support rod 36, a lower securing member 37, a secondary anchor insulation wall 32 and a primary anchor insulation wall 34, and the secondary sealing walls 23, 43 are connected to each other between the secondary anchor insulation wall 32 and the primary anchor insulation wall 34. The anchor support rod 36 is connected at one end thereof to the primary sealing wall 50 and is connected at the other end thereof to the inner wall 420 of the hull by the securing member 37.
The primary sealing wall 50 is welded to an upper end of the anchor support rod 36 of the anchor structure 30.
The anchor structure 30 is placed at a connection point between the neighboring planar structures 40 to interconnect the planar structures, and the planar structure 40 is secured to the inner wall 420 of the hull or the bulkhead 440, constituting the storage tank 10. In addition, the securing member 37 of the anchor structure 30 is disposed around the anchor support rod 36.
Next, another exemplary typical LNG storage tank will be described with reference to FIGS. 2 and 3. FIG. 2 is a view of an anchor member of another exemplary typical LNG storage tank, and FIG. 3 is a partial sectional view of the exemplary LNG storage tank.
Referring to FIG. 2, the anchor member 110 of the exemplary LNG storage tank has a truncated conical shape with a flat top surface. The truncated conical anchor member 110 has a closed upper surface and an open lower surface and has a body that widens toward an inner surface of the storage tank.
The anchor member 110 is formed under the conical body 111 with a securing portion 112 for securing the anchor member 110 to the inner surface of the storage tank, that is, an inner wall or a surface of a bulkhead.
In addition, the securing portion 112 has a plurality of through-holes 112a formed at regular intervals, such that a plurality of stud bolts 61 securely mounted on the inner surface of the storage tank 10 can be inserted into the respective through-holes 112a and then secured by nuts 62 (see FIG. 3).
Further, a step is formed above the conical body 111 of the anchor member 110 to form two joints 113, 114; a first sealing film 51 is securely mounted on the first joint 113 by welding; and a second sealing film 52 is securely mounted on the second joint 114 by welding.
In the typical LNG storage tank shown in FIG. 1, the heat insulating wall structure is composed of the primary and secondary heat insulating walls and the primary and secondary sealing walls and thus has a complicated structure. In addition, the structure for connecting the secondary sealing wall of the LNG storage tank is complicated, and installation of the heat insulating wall is not easy. Further, there is a possibility that sealing reliability of the secondary sealing wall is reduced (LNG leakage), since the structure of a connection of the anchor structure or the secondary sealing wall is complicated, and installation of the anchor structure or the secondary sealing wall is difficult.
In the typical LNG storage tank shown in FIGS. 2 and 3, since the joints and securing portion of the anchor member are not movable, it is difficult to cope with deformation of the hull and to eliminate stress concentration on the heat insulation system subjected to the load of LNG. In addition, there is a problem in that it is difficult to manufacture the anchor member due to the complicated shape thereof.